So what will be the fuel of choice for the cars of the future? Viera argued that it will be electricity, noting that Ford, in these lean economic times, is concentrating its research and development dollars in electric cars. "We recently decided to do electric cars because it is shorter term than hydrogen," he said at the Massachusetts Institute of Technology's Energy Conference on Saturday.

Sven Thesen of Better Place—a Palo Alto, Calif. purveyor of batteries and charging stations for electric cars —obviously agreed. Their plan is to bring a cell phone business model to the car industry in countries like Australia and Israel as well as states like Hawaii; that is, consumers buy their own car but are charged for the "minutes," which in this case would be kilowatt hours (racked up by recharging for quick trips or battery swapping for longer travel at stations akin to today's gas stations).

But there are three major problems with that vision, according to energy experts.

One: even though electrifying cars will reduce greenhouse gas emissions, it would either break the grid or require 500 new nuclear plants to charge the entire U.S. fleet of 200 million cars during the day, according to Daniel Snow, a technology professor at Harvard Business School. Not necessarily, said Thesen, who noted that 300 square miles of photovoltaics could do the trick. "You need an area one-fifth the size of Rhode Island covered in PV and you have your power source," he said.

Of course, PV doesn't produce power at night, which is when Thesen argued that such cars would have to be charged to avoid exponentially increasing peak electricity demand on, say, a hot, sunny, summer day. What's more: would electric car owners be willing to sell the juice in their batteries back to their neighbors (or the grid). "Am I going to use my Chevy Volt to light your lights? I don't think so," said Gary Colello, CEO of Premium Power, a battery maker in North Reading, Mass.

The second flaw: no one has built a production-ready electric car, though various conversions of the gas-electric hybrid Toyota Prius exist. Until the Chevy Volt or any of the numerous contenders (Better Place is said to be in talks with Nissan, which, like Ford, has also abandoned hydrogen in favor of electric) is in place, this model won't run.

Ultimately, Better Place may be selling electricity storage rather than electricity itself. After all, all those electric cars will each have a battery in them. Once enough such cars are in place they become a convenient storage place for electricity generated when it is not required, like the wind at night, a resource that is wasted at present.

The third problem is that electric cars may prove too expensive. According to Viera, the kinds of batteries required for true electric cars would add up to $15,000 to the base price of cars, too much for many consumers in the U.S. or the developing world. "That's going to be unaffordable," Viera said.

So it may turn out that improved internal combustion engines will continue to rule the road. There will be plenty of oil for them, predicted Atul Arya, BP's chief advisor on energy and climate policy, on the order of 2 trillion barrels in known reserves (although not all of that is readily accessible). But burning through all that even with engines that are 20 percent more efficient—something technically feasible and affordable today—the simple increase in the number of cars will more than outweigh any reduced greenhouse gas emissions, potentially leaving transportation an even bigger part of the climate change problem than it is now.

The only answer for that may be alternative liquid fuels: biofuels made from waste material like the cellulose in corn stalks rather than the starch in corn kernels. There's "500 million tons" of such material available in the U.S., research engineer Eric Larson of Princeton University said at the weekend conference. "That's biomass that does not have indirect land use impacts," like pushing aside food crops in favor of energy crops and placing the stomachs of the poor in competition with the gas tanks of the rich.

Such biofuels can render liquid transportation fuel "carbon neutral," because the amount of carbon dioxide emitted when they are burned is balanced by the amount of carbon dioxide used during the growth of the plant. Even so, it will be difficult for biofuels to replace a significant fraction of the oil used in cars today and, because it is more efficient and can even make electricity "carbon negative," it might make more sense to burn biomass with fossil fuels to produce electricity rather than turning it into fuel, according to Larson.

Politics will, of course, play a role in the fuels of the future. Representative Jay Inslee, D-Wash., predicted that measures to create a cap-and-trade scheme to restrict CO2 emissions, a mandate to derive some percentage of U.S. electricity from renewable resources and legislation to create a federal, so-called smart grid, would become law before the end of this year.

"It's an interesting race between electric cars and advanced biofuels," he told ScientificAmerican.com at the conference. "If we green the grid we can green the transportation sector."